Astigmatically corrected electronic lenses



y 10, 1956 A. ASMUS 2,754,443

ASTIGMATICALLY CORRECTED ELECTRONIC LENSES Filed Jan. 7, 1955 2Sheets-Sheet l ff .3 1a 19 y Int entan- July 10, 1956 A. ASMUSASTIGMATICALLY CORRECTED ELECTRONIC LENSES 2 Sheets-Sheet 2 Filed 195sInventor".-

United States fatent ASTIGMATICALLY CORRECTED ELECTRONIC LENSESAlexander Asmus, Berlin-Spandau, Germany, assignor to Siemens & HalskeAktiengesellschaft, Berlin-Siemensstadt, Germany, a German corporationApplication January 7, 1955, Serial No. 480,583

Claims priority, application Germany January 22, 1954 Claims. (Cl.313-84) My invention relates to electronic lenses, for instance as usedin electron microscopes, and is directed particularly to a compensatingdevice for correcting axial astigmatism in such lenses.

Even with highest-precision methods of manufacture, electronic lensescannot be produced with such accuracy as to avoid the occurrence ofelectron-optical lens errors due to departure of the magnetic orelectric lens field from perfect rotational symmetry. These errors,especialiy in objective lenses, have the result that the resolving powerof the electron optical apparatus does not attain the best expectedvalue. The lens errors are generally caused by unevenness in thesurfaces of the lensfield terminal structures such as the pole shoes orelectrodes of the lens and also by inhomogenities within the materialfrom which the lens is made. In such imperfect lenses, the intersectionsof the equipotential surfaces of the lens field with the planesperpendicular to the elec tron beam axis are not precisely circular butare approximately of elliptic shape. An error-free lens, however, mustnecessarily present precise circles as lines of intersection of theabove-mentioned equipotential surfaces and perpendicular planes.

It is accordingly the principal object of my invention to devise anelectron lens that readily permits correcting any residual error asmanifested by axial astigmatism.

Another object is to provide an electron lens with astigmatic correctionmeans that can readily be adjusted from the outside of an electronmicroscope or other electron-optical apparatus of which the lens forms acomponent.

To achieve these as well as the more particular objects mentioned below,and in accordance with a feature of my invention, 1 dispose around theelectron-optical axis and around the field of the lens an annularcorrective body for producing an elliptic field distortion adjustable asto magnitude and direction. According to another feature of theinvention, the corrective annular body is disposed so as to surround thefield-terminal system, i. e. the pole shoe or electrode systems, of thelens and is displaceable in the axial direction and also rotationallyadjustable about the lens axis. This makes it possible to correctastigmatism with the aid of only two continuously variable adjustments,namely axial displacement and rotation of the corrective member. Sincethe corrective member surrounds the major lens parts, namely the poleshoe system or the lens electrodes, depending upon whether the lens ismagnetic or electric, the desired lens correction is effected withoutnecessity of having any structural part enter into the lens fieldproper.

There are several ways, according to my invention, of designing thecorrective member. One way is to make the ring-shaped member of metaland give its inner periphery an asymmetrical configuration at one placeor at two diametrically opposite places. This can be done, for example,by recessing the ring-shaped member at these places.v Another. way 'ofproviding for the required "ice asymmetry, applying to magnetic lenses,is to make the corrective member of brass or other non-ferromagneticmaterial and to provide the member, for instance at two diametricallyopposite places, with iron or other ferromagnetic parts. Inelectrostatic lenses, the corrective member can be made ofnon-conducting material and be provided at suitable places withconducting metallic parts producing the desired field distortion.

According to another feature. of the. invention, the compensating devicepreferably has its ring-shaped corrective member provided with twocontrol shafts operable from the outside of the evacuated envelope. ofthe electron-optical apparatus, one shaft serving for rotationaladjustment and the other for adjusting axial displacement of the member.An especially simple construction is obtained by passing the two shaftscoaxially through a common bore in the vacuum wall of the lens assembly.

According to still another feature of the invention, the correctivemember, to be axially and rotationally adjustable, is mounted with theaid of two rings. One of these mounting rings has gear teeth meshingwith a pinion for rotational adjustment. The other ring has a screwthread which engages a threaded rotatable part and performs an axialdisplacement when the part is turned by another pinion. In a magneticlens this can be carried out by mounting a corrective iron member ofsuitable asymmetry between two brass rings, one having an outer, annularrow of gear teeth in mesh with a pinion for rotational adjustment of themember, while the other ring is externally threaded and engaged by arotatable, internally-threaded sleeve which in turn is rotatable bymeans of another pinion for thus imparting axial displacement to thecorrective member. According to a more specific feature, also relatingto magnetic lenses, one of the pole shoes of the lens to be correctedserves as a gliding surface for the corrective member.

These and other objects, features and advantages of the invention willbe apparent from the following description taken in conjunction with theaccompanying drawings, wherein:

Fig. 1 illustrates a vertical, partially cross-sectional view of amagnetic electron lens according to the invention;

Fig. 2 is a perspective view of the ring-shaped corrective member;

Fig. 3 shows the lower ring for turning the corrective member to controlthe radial angle at which distortion is introduced; and

Fig. 4 illustrates an alternative embodiment of the invention.

In Fig. 1, an upper pole shoe 1 and a lower pole shoe 2 form thefield-terminal structures of the lens. The magnetic circuit of the lenscomprises two annular cover plates 3 and 4, an outer cylindrical jacket5, and an inner cylindrical tube 6. An excitation winding 7 is disposedin the interior of the magnetic shield or capsule thus formed and isupwardly confined by an annular disc 8 of brass. A corrective member 9serves to correct the elliptical astigmatism of the lens. At twodiametrically opposite places, the member 9 is given suitableasymmetries capable of introducing a greater or lesser ellipticaldistortion of the field, depending upon the positioning of the member.To this end, in the embodiment of the invention illustrated, thering-shaped corrective member 9 has two diametrically opposed iron lugs10 and 11 serving as the zones of asymmetry. The ring-shaped correctivemember 9 itself may also consist of magnetic material but may also bemade of brass. Member 9 is mounted between two brass rings 12 and '13;and the entire subassembly of parts 9,. 12 and 13 is so shaped anddimensioned that the outer surface of the lower pole shoe 2 'serves .as'a gliding surface for that subassembly.

Two control shafts 14 and 15 serve for adjustment of the ring-shapedcorrective member 9. The shafts are coaxial and extend, one within theother, through the wall of the vacuum jacket 5. The central controlshaft 14 carries on its inner end a driving pinion meshing with anannular toothed portion 17 of ring 13. This drive mecha nism serves foradjusting the ring-shaped corrective member. 9 to rotate the correctivefield distortion.- The two' iron lugs and 11, fixed with respect to thecorrective member 9, project into complementary upper slots 18 and 19 inan upwardly extending collar 20 forming partjof ring 13, whereby thecorrective member is rotated the sleeve 23, the ring 12 is preventedfrom turning by V a pin 24 which is securely fixed at one end to theupper Wall. plate 3 of the lens and has its other end slidingly fittedinto a corresponding bore of ring 12. 'When turning the outer controlshaft 15, the sleeve 23 is rotated by pinion 21, and the ring 12 ismoved upwardly or downwardly and entrains the corrective member to movein the same axial direction; Turning the sleeve 23 in one direction thishas the effect of moving the corrective member 9 upwardly toward theeffective field gap of the lens so that the strength of corrective fielddistortion is increased. When turning the sleeve 23 in the otherdirection, a return spring 26 pushes the corrective member 9 downwardly,thus lessening the stength of field distortion.

If desired, the two control shafts 14 and 15 may extend .to the outsidethrough releasable and non-interchangeable clutch means. In any event,the'control knobs 27. and 28 of the two shafts should have fixed angularpositions relative to the respective shafts so that the knobs may serveas position-indicating means. 7 a

Fig. 4 illustrates schematically another embodiment of a correctivedevice for a magnetic lens. Only the essential parts of the pole shoesystem of the lens itself are shown. The pole shoe system comprises anupper pole shoe 3!, a lower pole shoe 32, and a non-magneticintermediate member 33 rigidly securing the pole shoes to each other. Aring-shaped corrective member 34 surrounding the pole shoe system isprovided for compensating axial astigmatism in the lens. Thecompensating member is made of brass and has a lower annular flange 35.The necessary asymmetry is provided by diametrically opposed iron lugs36 and 37 incorporated in the ring. For varying the direction ofcorrective field distortion, the corrective member 34 is cooperativelyconnected with a control shaft 37 carrying at its inner'end a driving 7pinion 38 meshing with an annular toothed portion of a brass ring 39. Awedged-shaped brass. lug 41 fixed with respect to the brass ring 39, orintegrated therewith and projecting inwardly, is fitted within acomplementary notch 42 of the ring-shaped corrective 34 and is slidableup and down therein. The brass lug 41, upon turning, carries with it thering-shaped corrective member 34 and turns it about the pole shoe systemfor changing the direction of field distortion.

For varying the strength of field distortion by axial adjustment of theringeshaped corrective member 34, the lens system comprises a controlshaft 43 which carries on its inner end a driving pinion 44 meshing withan annular toothed portion 45 of a positioning ring 46. The'turningmovement of this positioning ring is imparted to a ring member 49through two downwardly-extending pins 47, 48 each fixed at one end inpositioning ring 46 and fitting slidably at their other ends incomplementary openings in the ring member 49; The ring member 49 has an'inner annular groove 5% within which the peripheral edge of a flangeportion of the ring-shaped corrective member 34 is seated. The ringmember 49 has an external screw thread 51 screwed within aninternally-threaded fixed member 52, whereby, when turning the ringmember 46, the ring member 49 will be screwed upwardly or downwardly inthe internally-threaded member 52. Axial movement is thereby imparted tothe corrective member 34 thus varying the strength of field distortion.By suitable proportioning of the frictional conditions, the correctivemember 34 is prevented from being carried along by the turning motion ofthe ring member 4?. The outer control knobs 53 and 54 in this embodimentof the invention are fixed to the ends of the respective control shafts37 and 43, which extend through the vacuum wall (not shown in Fig. 6) tobe outside of the apparatus.

It will be apparent to those skilled in the art upon study of thisdisclosure thatthe invention permits of various embodiments,modifications and uses other than those specifically described hereinwithout departing from the essential features of the invention andWithin the scope of the claims annexed hereto.

I claim:

1. An electron-optical lens, comprising annular lensfield terminalstructures fixed and coaxially spaced relative to each other anddefining a lens field and a lens axis, a corrective body of annularshape surrounding said structure, said corrective body beingasymmetrically designedat diametrically opposed locations forelliptically distorting the lens field, said annular body being axiallydisplaceable relative to said structure and being rotatably adjustableabout the lens axis for compensating .axial astigmatism.

2. An electron-optical lens, comprising two annular magnet pole shoesfixed and coaxially spaced fromeach other to form a lens-field gapdefining a lens axis, a corrective body of annular shape having a largerinner diameter than said pole shoes and surrounding said gap, saidcorrective body being asymmetrically designed at diametrically opposedlocations for elliptically distorting the lens field, said annular bodybeing axially displaceable relative to said structure and beingrotatably adjustable about the lens axis for compensating axialastigmatism.

3. An electronic lens, comprising a vacuum envelope, annular lens-fieldterminal structures within said envelope coaxially spaced from eachother and defining a lens field and a lens axis, an annular correctivememberdisposed in said envelope and surrounding said axis, saidcorrective member being asymmetrically designed at diametrically opposedlocations for elliptically distorting the lens field, andmanually-controllable drive means'for independently varying the axialand rotary positions of said corrective 'member with respect to saidaxis, said drive means extending from within to the outside of saidenvelope.

'4. An electronic lens, comprising annular lens-field terminalstructures coaxially spaced relative each other and defining a lens'field and lens axis, an annular corrective member surrounding said axis,said corrective member being asymmetrically designed at diametricallyopposed locations for elliptically distorting the lens field, a firstdrive mechanism for controlling the axial position of said correctivemember along saidaxis, and a second drive mechanism for controlling therotary position of said corrective member with'respect to said axis.

5. An electronic lens, comprising a vacuum envelope, annular lens-fieldterminal structures coaxially spaced from each other within saidenvelope and defining a lens field and a lens axis, an annularcorrective member disposed in said envelope surrounding said axis, saidcorrective member being asymmetrically designed at diametrically opposedlocations for elliptically distorting the lens field, a first drivemechanism for controlling the axial position of said corrective memberalong said axis, and a second drive mechanism for controlling therotaryposition of said corrective member with respect to said axis,

said first and second drive mechanisms being disposed within saidenvelope and comprising a pair of manually operable drive shafts, one ofsaid shafts being tubular and having the other of said shafts coaxiallydisposed therein, and said two shafts extending from Within to theoutside of said envelope.

6. An electronic lens, comprising annular lens-field terminal structurescoaxially spaced relative each other and defining a lens field and alens axis, an annular corrective member surrounding said axis, saidcorrective member being asymmetrically designed at diametrically opposedlocations for elliptically distorting the lens field, anexternally-threaded ring member coaxial With said corrective member androtatable with respect thereto, a rotatable internally-threaded memberWithin which said externally-threaded member is threaded, saidinternally-threaded member having a first annular gear portion, a firstdrive gear in mesh with said first annular gear portion for turning saidinternally threaded member, a second ring member coaxial with saidcorrective member, means interconnecting said second ring member andsaid corrective member for simultaneous cooperative rotary motion, saidsecond ring member having a second annular gear portion, and a seconddrive gear in mesh with said second annular gear portion for turningsaid second ring member.

7. An electronic lens, comprising two fixed magnet pole shoes coaxiallyspaced from each other to form a lensfield gap defining a lens axis, acorrective member of annular shape having a larger inner diameter thanthe outer diameter of said pole shoes and surrounding said gap, saidcorrective member being asymmetrically designed at diametrically opposedlocations for elliptically distorting the lens field, a first drivemechanism for controlling the axial position of said corrective memberalong said axis, and a second drive mechanism for controlling the rotaryposition of said corrective member With respect to said axis.

8. An electronic lens, comprising two fixed magnet pole shoes coaxiallyspaced from each other to form a lensfield gap defining a lens axis, oneof said pole shoes having an annular outer slide surface, a correctivemember of annular shape having a larger inner diameter than the outerdiameter of said pole shoes and surrounding said gap, said correctivemember being asymmetrically designed at diametrically opposed locationsfor elliptically distorting the lens field, holding means seated uponsaid slide surface and connected with said corrective member wherebysaid member is rotatable and axially displaceable relative to said slidesurface, a first drive mechanism engaging said holding means forcontrolling the axial position of said corrective member along saidaxis, and a second drive mechanism engaging said holding means forcontrolling the rotary position of said corrective member with respectto said axis.

9. An electronic lens, comprising two fixed magnet pole shoes coaxiallyspaced from each other to form a lensfield gap defining a lens axis, a.corrective member of annular shape having a larger inner diameter thanthe outer diameter of said pole shoes and surrounding said gap, saidcorrective member being asymmetrically designed at diametrically opposedlocations for elliptically distorting the lens field, anexternally-threaded ring member coaxial with said corrective member androtatable with respect thereto, a rotatable internally-threaded memberwithin which said externally-threaded member is threaded, saidinternally-threaded member having a first annular gear portion forturning said internally theaded tember, a second ring member coaxialwith said corrective member, means interconnecting said second ringmember and said corrective member for simultaneous cooperative rotarymotion, said second ring member having a second annular gear portion,and a second drive gear in mesh with said second annular gear portionfor turning said second ring member.

l0. An electronic lens, comprising two fixed magnet pole shoes coaxiallyspaced from each other to form a lens-field gap defining a lens axis, acorrective member of annular shape having a larger inner diameter thanthe outer diameter of said pole shoes and surrounding said gap, saidcorrective member being asymmetrically designed at diametrically opposedlocations for elliptically distorting the lens field, anexternally-threaded annular member coaxial with said corrective memberand rotatable with respect thereto, means interconnecting saidexternally-threaded member and said corrective member for simultaneousaxial motion, a fixed, internally-threaded member within which saidexternally-threaded member is threadedly engaged, a first ring membercoaxial with said corrective member and having a first annular gearportion, relative axially slidable means interconnecting said first ringmember and said externallythreaded member for simultaneous rotarymotion, a first manually-operable drive gear in mesh with said firstannular gear portion for turning said first ring member, a second ringmember coaxial with said corrective member and having a second annulargear portion, means interconnecting said second ring member and saidcorrective member for simultaneous rotary motion and allowing relativeaxial motion, and a second manually operable drive gear in mesh withsaid second annular gear portion for turning said second ring member andsaid externallythreaded annular member.

References Cited in the file of this patent UNITED STATES PATENTS2,219,193 Mynall Oct. 22, 1940 2,416,687 Fry Mar. 4, 1947 2,5 87,942Wessenberg et al Mar. 4, 1952 2,637,000 Page Apr. 28, 1953 2,679,018Reisner et al May 18, 1954 2,714,678 Wolff Aug. 2, 1955

